1. Field of the Disclosure
The present disclosure relates to an organic light emitting display device, and more particularly, to an organic light emitting display device and a method of driving the same, which can increase accuracy and stability in compensating for deterioration of a driving thin film transistor (TFT).
2. Discussion of the Related Art
FIG. 1 is a circuit diagram for describing a pixel structure of a related art organic light emitting display device.
Referring to FIG. 1, the related art organic light emitting display device includes a display panel in which a plurality of pixels are formed. Each of the pixels includes a first switching TFT ST1, a second switching TFT ST2, a driving TFT DT, a capacitor Cst, and an organic light emitting diode OLED.
The first switching TFT ST1 is turned on according to a scan signal (gate driving signal) supplied to a corresponding gate line GL. The first switching TFT ST1 is turned on, and thus, a data voltage Vdata supplied to a corresponding data line DL is supplied to the driving TFT DT.
The driving TFT DT is turned on with the data voltage Vdata supplied to the first switching TFT ST1. A data current Ioled flowing to the organic light emitting diode OLED is controlled with a switching time of the driving TFT DT. A first driving voltage VDD is supplied to a power line PL, and, when the driving TFT DT is turned on, the data current Ioled is applied to the organic light emitting diode OLED.
The capacitor Cst is connected between a gate and source of the driving TFT DT. The capacitor Cst stores a voltage corresponding to the data voltage Vdata supplied to the gate of the driving TFT DT. The driving TFT DT is turned on with the voltage stored in the capacitor Cst.
The organic light emitting diode OLED is electrically connected between the source of the driving TFT DT and a cathode voltage VSS. The organic light emitting diode OLED emits light with the data current Ioled supplied from the driving TFT DT.
The related art organic light emitting display device controls a level of the data current Ioled flowing from a first driving voltage VDD terminal to the organic light emitting diode OLED with a switching time of the driving TFT DT based on the data voltage Vdata. Therefore, the organic light emitting diode OLED of each pixel emits light, thereby realizing an image.
However, the threshold voltage (Vth) and mobility characteristics of the driving TFTs DT of the respective pixels are differently shown due to a non-uniformity of a TFT manufacturing process. For this reason, in general organic light emitting display devices, despite that the same data voltage Vdata is applied to the driving TFTs DT of the respective pixels, it is unable to realize a uniform image quality due to a deviation of currents flowing in the respective organic light emitting diodes OLED.
To solve a non-uniformity of an image quality, the second switching TFT ST2 is additionally formed in each pixel. The second switching TFT ST2 is turned on according to a sensing signal applied to a corresponding sensing signal line SL. The second switching TFT ST2 is turned on, and thus, the data current Ioled supplied to the organic light emitting diode OLED is supplied to an analog-to-digital converter (ADC) of a data driver. A plurality of the sensing signal lines SL are formed in the same direction as that of the gate line GL.
FIG. 2 is a diagram illustrating a method of compensating for a characteristic deviation of the driving TFTs in the related art organic light emitting display device.
Referring to FIG. 2, the display panel has been manufactured, and then, before a product is released, the second switching TFTs ST2 of all the pixels are turned on, and a voltage charged into each of a plurality of reference power lines RL is sensed, in operation S1. Subsequently, the compensation method generates sensing data corresponding to the sensed characteristics (threshold voltage/mobility) of the driving TFTs DT of all the pixels.
Subsequently, the compensation method generates initial compensation data on the basis of the sensing data, and initially compensates for the characteristics (threshold voltage/mobility) of the driving TFTs DT of all the pixels with the initial compensation data.
After the initial compensation, when the display panel has been released as a product, real-time sensing is performed. The compensation method selectively turns on the second switching TFTs ST2 of a plurality of pixels arranged on one horizontal line during a blank interval between frames to sense a voltage charged into each reference power line RL in real time while displaying an image, in operation S3.
Subsequently, the compensation method converts the sensed voltage into compensation data corresponding to the characteristic (threshold voltage/mobility) of the driving TFT DT of each pixel. The compensation method compensates the characteristic of the driving TFT with the compensation data, in operation S4.
Subsequently, the compensation method checks whether the organic light emitting display device is powered off in operation. S5, and, when the organic light emitting display device is not powered off, the compensation method repeats operations S3 to S5 to compensate for the characteristics of the driving TFTs of all the pixels in real time.
However, when the organic light emitting display device is driven for a long time, there is a limitation in measuring a characteristic deviation of the pixels to compensate for the characteristic deviation in real time.
Specifically, a range for sensing the characteristic of each driving TFT and a range of compensation data are decided according to an output range of each of the ADCs of the data driver. It is difficult to expand the output range of each ADC of the data driver, and for this reason, there is a limitation in range of compensating for a deviation of the driving TFTs at one time through real-time sensing.
Moreover, when a change amount of characteristic of each driving TFT is large due to long-time driving, it is unable to all sense the changed characteristics and compensate for the sensed changes at one time, and thus, it is required to perform sensing and compensation driving several times. Especially, when the characteristic of each driving TFT deviates from a range of a corresponding ADC, it is unable to accurately sense a change in characteristic of each driving TFT, and thus, an accuracy of compensation decreases.
In real-time sensing and compensation driving, since sensing and compensation are performed during the blank interval while displaying an image, an error of a sensing value occurs due to a data voltage supplied to each pixel for displaying an image immediately before sensing.
Moreover, since a real-time sensing scheme is sensitive to an influence of an ambient environment (for example, temperature), there is a high possibility that an error of sensing data occurs.
Moreover, when sensing and compensation driving are performed in several stages, a user can perceive a sensing line, and a luminance difference occurs between pixels under compensation and other pixels, causing a degradation of a display quality.
To solve such problems, the range of each ADC may be greatly set. However, when a compensation range of each ADC is large, compensation of each pixel may be performed at a fast speed, but in this case, an influence of a noise increases. As the range of each ADC is expanded, a sensing range and a compensation range are expanded together, and an accuracy of sensing decreases. Furthermore, since a large compensation value is reflected at one time, a user perceives a change in luminance.